feat(app): redesigned interface, polls, drafts, forwarding and mentions

A new look across the app and a set of everyday messaging features.

Interface
- Home screen with Chats, Contacts and Settings tabs, a large title,
  search across chats and message text (diacritics-insensitive), filters
  and pinned chats.
- Conversation view with a translucent header and composer, grouped
  bubbles, author avatars and colours in groups, swipe to reply, an
  "N new messages" marker, a jump-to-latest button and in-chat search.
- Contact and group profiles: verification, shared photos, groups in
  common, disappearing messages, mute, block; members, admin badges,
  adding people, rename and leave for groups.
- Full-screen photo viewer that pages through the whole conversation.
- A stable avatar colour per person and a clearly visible read state.

Messaging
- Polls in chats and groups, single or multiple choice, with votes that
  can be changed or withdrawn; older clients see them as text.
- Drafts survive leaving a chat and show in the chat list.
- Forwarding of messages, photos, files and voice messages.
- Notes to self: a local-only, encrypted chat that never reaches the
  relay.
- @mentions in groups that notify even when the group is muted.
- Typing indicators sent as ephemeral messages, opt-in and mutual like
  read receipts.
- Delivery and read times per message and per group member.
- Profile photos, cropped, resized and stripped of metadata, delivered
  encrypted to contacts.
- Voice message playback at 1.5x and 2x.
- Call buttons in 1:1 chats and call log entries.

Fixes
- Muted groups no longer notify: muting was checked against the
  author's chat instead of the group.
- A blocked contact could unblock itself by changing its delivery
  address.
- Read receipts no longer regress to delivered when a late delivery
  receipt arrives.
- A message marked as failed corrects itself once it is delivered.
- Double ticks in groups now mean delivered to every member.

app/lib/models/conversation.dart

@@ -9,10 +9,11 @@ class Conversation {
   DateTime lastActivity;
   int? disappearAfterSeconds; // Globální nastavení mizících zpráv
 
-  /// Ztlumení a archivace — čistě LOKÁLNÍ (neopouští zařízení).
+  /// Ztlumení, archivace a připnutí — čistě LOKÁLNÍ (neopouští zařízení).
   bool muted;
   DateTime? mutedUntil; // null + muted = navždy; jinak dočasně
   bool archived;
+  bool pinned; // připnutý nahoře v seznamu chatů (lokální)
 
   Conversation({
     required this.contactId,
@@ -22,6 +23,7 @@ class Conversation {
     this.muted = false,
     this.mutedUntil,
     this.archived = false,
+    this.pinned = false,
   })  : messages = messages ?? [],
         lastActivity = lastActivity ?? DateTime.now();
 
@@ -75,6 +77,7 @@ class Conversation {
         'muted': muted,
         'mutedUntil': mutedUntil?.toIso8601String(),
         'archived': archived,
+        'pinned': pinned,
       };
 
   factory Conversation.fromJson(Map<String, dynamic> json) {
@@ -92,6 +95,7 @@ class Conversation {
           ? null
           : DateTime.parse(json['mutedUntil'] as String),
       archived: json['archived'] as bool? ?? false,
+      pinned: json['pinned'] as bool? ?? false,
     );
   }
 }